Bio-fuel furnace

ABSTRACT

This bio-fuel furnace includes a combustion unit and separate fuel feed unit mounted atop the combustion unit. The combustion unit includes a cylindrical burn pot, a preheat module, an expansion bowl and a primary heat exchanger supported by an inner frame and enclosed by an outer casing. The preheat module is a rectangular box having an internal spiral baffle. The burn pot is seated within a central opening in the preheat module. The expansion bowl is seated atop the preheat module and the primary heat exchanger is mounted above the expansion bowl. An inlet fan blows inlet air through the preheat module and into the burn pot and an outlet fan draws exhaust air around the primary heat exchanger from the combustion unit. The fuel feed unit has an internal storage bin and a rotating scoop arm deposits the bio fuel into the combustion unit. The rotation of the scoop arm meters the bio-fuel into the combustion unit, which falls vertically directly into the burn pot though a center passage in the primary heat exchanger and the expansion bowl.

This invention relates to furnaces for burning shredded, granulize, extruded or pelletized bio-fuels in residential, commercial and industrial HVAC and energy production applications.

BACKGROUND OF THE INVENTION

The term “bio-fuel” refers to renewable energy sources obtained from various living or recently living biological materials, but generally excludes fossil fuels, which are also organic materials that have been transformed through geological process. Although they have their origin in ancient biological matter (“biomass”), they are not considered bio-fuels by the generally accepted definition because they contain carbon that has been “out” of the carbon cycle for a very long time. Plant and animal by-products can all be used as bio-fuels. Wood, forest residue, sawdust, yard clippings, domestic refuse, agricultural waste, shelled corn, non-food energy crops, animal fats, and dried manure can all be used as solid bio-fuels.

The burning of solid bio-fuel to generate heat began with the discovery of fire. Cut and split wood logs have been burned in furnaces for centuries and wood remains the most recognized and readily used type of solid bio fuel for heat production. Firewood and wood pellet furnaces are well known and commonly used for domestic heating applications. Conventional firewood and pellet furnaces use a fire box where the firewood or wood pellets are burned. Firewood furnaces are extremely labor intensive. In addition to cutting and splitting the firewood, in most cases, firewood furnaces must be manually stoked. While firewood furnaces must be manually stoked, pellet furnaces use an auger to deposit the fuel pellets into the firebox from a storage bin or hopper.

Heretofore, solid bio-fuel furnaces including wood pellet furnaces have had many disadvantages in terms of fuel combustion. Conventional bio-fuel furnaces are limited to the types of bio-fuels that can be burned. Wood pellet furnaces in particular, have difficulty burning solid bio-fuel with high moisture content, which is generally more than fifteen percent (15%) moisture content, such as shell corn, which can have a moisture content far exceeding fifteen percent. Normally, burning “dry” bio-fuels produces flame temperatures ranging between 900-1600° F. When bio-fuels having a high moisture content, that is generally exceeding fifteen percent (15%), are burned in conventional pellet type furnaces, steam is produced, which quickly reduces the temperature of the flame below the combustion temperature of the bio-fuel thereby extinguishing the flame.

In addition, conventional bio-fuel furnaces are limited to the amount of non-combustible materials that can be mixed with the bio-fuel. Noncombustible contaminants in the bio fuel can include sand, small rocks and gravel, metal scraps, glass and other inorganic material. Generally, conventional bio-fuel furnaces are limited to no more than a three percent (3%) mix of non-combustible materials in the bio-fuel. Non-combustible materials not only reduce the efficient combustion of conventional bio-fuel furnaces but can also damage the fuel feed augers, burn trough and other components. In the combustion process, at temperatures below 1600° F., non-combustible materials combine chemically to the non-combusted fuel particles resulting in a “clinker,” which is a lump of un-combusted material. Clinkers block air-flow greatly reducing combustion efficiency and can eventually extinguish the flame.

Conventional solid bio-fuel furnaces are also plagued with certain practical inconveniences and mechanical issues. Most conventional bio-fuel furnaces have an ash collection container where the combustion by-products (ash) collects. This container must be manually emptied of embers and ash on a regular basis. Conventional bio-fuel furnaces tend to have small fuel storage bins, which require frequent replenishing. A typical gravity feed storage bin may only hold twenty to forty pounds (20-40 lbs.) of wood pellets.

Auger mechanisms used in many conventional pellet type furnaces are the source of many additional mechanical drawbacks and inefficiencies. Inside the firebox, the auger generally deposits the pelletized fuel atop a tray or trough where it is burned, but the air flow within the firebox and particularly around the burn trough is insufficient to completely or efficiently combust the bio fuel. The auger itself often restricts airflow around the burn tray. As a result, the combustion process within conventional bio-fuel furnaces generates large amounts of ash and consumed fuel, as well as excess exhaust gases. Thermal expansion of the metal auger tube due to the high temperatures inside the firebox can cause the auger tube to bend and deform, which results in abrasion wear from the contact with the auger. Augers also tend to grind the pelletized fuel into powder which clogs the auger tube. Augers cannot move this powder and the powder gradually adheres to the inside of the auger tube, which restricts the flow of bio-fuel through the auger tube. Since the augers generally pull fuel from the bottom of the storage bins, the weight of the pellet fuel rests atop the auger. The weight of the fuel causes excess strain on the auger motor and excess wear on the auger blades.

SUMMARY OF THE INVENTION

The present invention seeks to provide a bio-fuel furnace that can burn any solid bio-fuel in a shredded, granularized or pelletized form for use in producing heat, hot water or steam in residential, commercial and industrial HVAC and other energy production applications. The bio-fuel furnace embodying the present invention includes a combustion unit and separate fuel feed unit mounted above the combustion unit. The combustion unit includes a cylindrical burn pot, a preheat module, an expansion bowl and a primary heat exchanger supported by an inner frame and enclosed by an outer casing. The preheat module is a rectangular box having an internal spiral baffle. The burn pot is seated within a central opening in the preheat module. The expansion bowl is seated atop the preheat module and the primary heat exchanger is mounted above the expansion bowl. An inlet fan blows inlet air through the preheat module and into the burn pot and an outlet fan draws exhaust air around the primary heat exchanger from the combustion unit. The fuel feed unit has an internal storage bin and a rotating scoop arm deposits the bio-fuel into the combustion unit. The rotation of the scoop arm meters the bio-fuel into the combustion unit, which falls vertically directly into the burn pot though a center passage in the primary heat exchanger and the expansion bowl.

The bio furnace of this invention combusts bio-fuel by first preheating an inlet airflow within the preheat module. The preheated inlet air is then vented into the burn pot through a plurality of angled holes in the burn pot, which creates a combustion vortex of swirling inlet air, combusting gases and thermal energy within the burn pot. The bio-fuel is metered into the combustion unit vertically through the combustion vortex where it is burned. As the combustion vortex rises above the burn pot, it expands in the expansion area to disperse the thermal energy across the bottom of the primary heat exchanger before being vented from the combustion unit.

Accordingly, the bio-fuel furnace of this invention has the ability to burn a wider range of bio-fuels. As a result of preheating the inlet air flow and creating a combustion vortex within which the bio-fuel is burned, the furnace burns so efficiently and at such high combustion temperatures that even bio-fuel with high moisture content can be burned without extinguishing the flame. The furnaces of this invention can burn bio-fuels with moisture content above twenty-five percent (25%). In addition, the furnace can burn bio-fuels with a high content of inert and non-combustible contaminants.

The preheat module uses a portion of the thermal energy from the combustion of bio-fuel in the burn pot to preheat the inlet air flow to temperatures well above the ignition point of the bio-fuel. The angled holes in the burn pot directionally vent the inlet airflow into the burn pot where the combustion of the bio-fuel occurs. The spiraling air flow surrounds and envelopes the bio-fuel, which falls vertically through the combustion vortex. Consequently, the preheated oxygen rich inlet air spiraling in the combustion vortex provides an ideal environment for efficient and complete combustion. The combustion vortex is so strong that the falling bio fuel is fully ignited before it settles at the bottom of the burn pot. Because the combustion is so efficient, only elemental ash and gases are produced. Any inert matter is carried upward within the combustion vortex and vented from the combustion unit with the exhaust air. The directional venting of the inlet airflow into the burn pot also creates an insulating swirling curtain of inlet air along the inside of the burn pot, which act as a protective thermal barrier centering the combustion vortex and spacing the superheated combustion vortex from the sidewalls of the burn pot.

The furnace can be readily adapted for use with any conventional HVAC system in any heating or energy production application. The furnace uses a modular design, which allows for the use of standard fittings, connectors, fans, motors, switches, wiring and controllers and allows for individual components and parts to be readily replaced, repaired or adapted for different applications. Nearly all of the components, modules and assemblies can be repaired or replaced by users with basic tools and without specialized knowledge or skill. The feed unit with the rotating scoop arm mechanism provides a maintenance free fuel delivery mechanism and eliminates the many problems of conventional auger fed bio-fuel furnaces.

Theses and other advantages of the present invention will become apparent from the following description of an embodiment of the invention with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate an embodiment of the present invention, in which:

FIG. 1 is a perspective view of an embodiment of the bio-fuel furnace of this invention;

FIG. 2 is an exploded view of the bio-fuel furnace of FIG. 1;

FIG. 3 is an exploded view of the combustion unit of the bio-fuel furnace of FIG. 1;

FIG. 4 is a side view of the preheat module of the bio-fuel furnace of FIG. 1;

FIG. 5 is a top sectional view of the preheat module and burn pot of FIG. 4 taken along line 5-5;

FIG. 6 is a partial top sectional view of the preheat module and burn pot of the bio-fuel furnace of FIG. 1;

FIG. 7 is a perspective view of the primary heat exchanger of the bio-fuel furnace of FIG. 1;

FIG. 8 is an exploded perspective view of the primary heat exchanger of the bio-fuel furnace of FIG. 1 with a portion cutaway to show the internal baffles;

FIG. 9 is a simplified perspective view of the electronic controller, fans and scoop arm drive motors used in the bio-fuel furnace of FIG. 1;

FIG. 10 is an exploded view of the fuel feed unit of the bio-fuel furnace of FIG. 1;

FIG. 11 is a partial exploded view with portions cutaway of the preheat module, burn pot, expansion bowl and heat exchanger of FIG. 1 showing the bio-fuel and inlet air flow entering the burn pot;

FIG. 12 is a front view with sections cutaway of the combustion unit of FIG. 1;

FIG. 13 is a side sectional view of the combustion unit of FIG. 1 showing the air flow through the combustion unit;

FIG. 14 is a side sectional view of the combustion unit of FIG. 1 showing the air flow vortex within the burn pot and expansion bowl;

FIG. 15 is an exploded view of an alternative embodiment of the combustion unit;

FIG. 16 is a side sectional view of the combustion unit of FIG. 15; and

FIG. 17 partial perspective view with sections cutaway of the preheat module, burn pot, expansion bowl, heat exchanger and the by-product handling assembly of FIG. 15.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, FIGS. 1-14 illustrate an embodiment of the bio-fuel furnace of this invention, which is designated and identified as reference number 10. As shown, furnace 10 includes two main components, a combustion unit 20 and separate fuel feed unit 70 mounted atop the combustion unit. As a separate component, fuel feed unit 70 can be detached from combustion unit 20 for transport and repair.

As shown best in FIG. 3, combustion unit 20 includes several internal components and modules, namely, a burn pot 30, a preheat module 40, an expansion bowl 50 and a primary heat exchanger 60 supported by an inner frame 24 and enclosed by an outer casing 22. Outer casing 22 is generally constructed of sheet metal, although any suitable material may be used. Combustion unit 20 also includes a thermal lining 23 of autoclaved aerated concrete (AAC) or cellular concrete (CC). Preformed bricks and custom formed pieces of AAC or CC line the inside of combustion unit 20 between the inner frame 24 and outer casing 22. Inner frame 24 has a box shape with an open front and includes an elevated platform 26 upon which the internal components and modules are stacked and supported. As shown, elevated platform 26 is spaced above the thermally lined bottom of outer casing 22. A door 28 on the front panel of outer casing 22 provides access to the interior of combustion unit 10 and the various components and modules supported by inner frame 24. Combustion unit 20 has a vertical openings 21 through the top of outer casing 22, inner frame 24 and thermal lining 23, which forms a central fuel passage that passes through openings in heat exchanger 60 into expansion bowl 50 through which bio-fuel falls into burn pot 30.

Burn pot 30 is formed or cast from a durable thermal conducting material, such as cast iron, steel or carbon fiber. As will be explained hereafter, burn pot 30 does not need to be constructed from costly high temperature materials that have high melt points in order to withstand the extremely high combustion temperatures that are produced within the burn pot. Burn pot 30 has cylindrical walls and a domed bottom, which forms an open topped cylindrical interior 31. A plurality of vent holes 33 are formed in the cylindrical walls and domed bottom of the burn pot. As shown in FIG. 6, vent holes 33 in the wall and bottom of burn pot 30 are bored or formed to extend laterally through the wall at approximately a 30° angle to the wall's radius.

Preheat module 40 is seated directly atop platform 26. Preheat module 40 is a rectangular box constructed of metal, carbon fiber or other material with high thermal conducting properties. Preheat module 40 has a central cylindrical opening 41 within which burn pot 30 is seated. As best shown in FIGS. 4 and 11, preheat assembly 40 includes an internal baffle 42 that spirals around burn pot 30. A domed cover 44 encloses the bottom of opening 41 and is spaced from the domed bottom of burn pot 30. Cover 44 is secured to the bottom of preheat module by fasteners (not shown). Typically, a high temperature gasket (not shown) is used to provide a substantially air tight seal between the bottom of the preheat module 40 and the cover 44. The bottom of preheat module 40 also has a vent slot 45 formed by a bent flange 46 which vents inlet air between cover 44 and the bottom of burn pot 30. An electric inlet fan 92 mounted to the back of outer casing 22 blows air into preheat module 40 through an inlet duct 48, which extends through openings in inner frame 24 and outer casing 22. Inlet fan 92 may be of any conventional design to provide a controlled and variable airflow into the preheat module.

Expansion bowl 50 sits atop preheat module 40 within inner frame 24. Expansion bowl 50 is formed or molded from AAC, CC or other suitable thermal insulating material. As shown, expansion bowl 50 has a box shaped body with a central opening 51 and a concave top surface 52, which forms a combustion expansion area 53 for rapidly expanding gases above burn pot 30. The concave top surface of expansion bowl 50 also has a recessed corner furrow 55 that forms an exhaust passage. Furrow 55 opens into the mouth of an L-shaped exhaust duct 58 mounted to the back of inner frame 24. An exhaust fan 94 also mounted to the back of outer casing 22 draws exhaust air from the expansion area 53 through furrow 55 and exhaust duct 58. Again, exhaust fan 94 is of any conventional design, which draws a controlled and variable airflow from expansion area 53.

Primary heat exchanger 60 is mounted within inner frame 24 above expansion bowl 50. Heat exchanger 60 is an enclosed rectangular box constructed of metal, carbon fiber or other material with high thermal conducting properties. Heat exchanger 60 includes two threaded ports 62, which are used to connect furnace 10 to the heat distribution system. Heat exchanger 60 is designed to be bi-directional and compatible with both forced air and hydronic distribution systems. As such, any transfer medium can be circulated through heat exchanger 60 in any direction. It should be noted that ports 62 are standard plumbing fittings, which allow simple connection to any conventional heat distribution systems without special tooling or connectors. As shown in FIG. 8, heat exchanger 60 includes a plurality of internal baffles 64, which provide more surface area through which thermal energy can be conducted into the exchange media. Heat exchanger 60 has a central passage 61, through which fuel falls into the combustion area of burn pot 30 through expansion area 51 of the expansion bowl 50.

Fuel feed unit 70 includes an outer casing 72 constructed generally of sheet metal or other suitable material. The interior of fuel feed unit 70 is divided into a hopper area 73 and a metering area 75 by an interior partition wall 74. The bottom of partition wall 74 is spaced above the unit bottom so that bio fuel flows under the wall from hopper area 73 into metering area 75. A door 76 on the front of outer casing 22 provides access to metering area 75. A feed tube 78 extends upward at an angle from the unit bottom in metering area 75. Feed tube 78 openings into the central fuel passage of combustion unit 20. A rotating scoop arm 80 is mounted to the front of partition wall 74, which meters bio-fuel at the bottom of the metering area 75 into feed tube 78 Scoop bucket 82 is formed or connected to the distal end of scoop arm 80. Scoop arm 80 is turned by an electric motor 96 mounted to partition wall 74. Drive motor 96 is of conventional design and selected to provide sufficient power to rotate scoop arm 80 and pass scoop bucket 82 through the bio-fuel in the bottom of metering area 75. As shown, a contact switch 98 mounted to partition wall 74 and wired to controller 90 is used to interrupt the rotation of scoop arm 80 and control metering of the bio fuel. Contact switch 98 is positioned so that scoop arm 80 trips the switch with each revolution.

The operation of inlet fan 92, exhaust fan 94 and scoop arm motor 96 is controlled by an electronic controller 90 mounted to the back of combustion unit 20. Controller 90 uses conventional circuitry and is designed to connect to and interface with conventional thermostats to control the operation of furnace 10. Generally, DC drive motors and fans and control circuitry are used, but AC motors and fans can be used within the teachings of this invention. DC motors and circuitry allow furnace 10 to be powered from low voltage power sources, such as batteries and solar cells, as well as from AC line power using readily available transformers.

FIGS. 11-14 illustrate the basic operation of furnace 10 burning a typical bio-fuel 2. All forms of biomass can be burned as a bio-fuel in furnace 10, including, wood, forest residue, sawdust, paper, yard clippings, domestic refuse, agricultural waste, shelled corn, fatty oils, non-food energy crops, plastics and dried manure, etc. . . . . The biomass needs only to be shredded, granulated or pelletized to the degree that it will flow under wall from hopper 73 and be metered from feed unit 70 into combustion unit 10. Furnace 10 can efficiently burn bio-fuels with moisture content in excess of twenty-five percent (25%). Furnace 10 will efficiently burn any combination or mixture of biomass. For example, furnace 10 will burn bio-fuel consisting of shredded and pelletized paper with a very low moisture content, as well as, pelletized grass clippings having a higher moisture content, pelletized fatty oils can be burned alone or in combination with shredded tree branches, or shredded plastics can be burned with other shredded office waste products.

Furnace 10 combusts the bio-fuel by first preheating the inlet airflow 4 to more than twice the combustion temperature of the bio-fuel and venting that preheated airflow into burn pot 30 to create a combustion vortex 6 of swirling inlet air, combusting gases and thermal energy within burn pot 30. Bio-fuel 2 is metered into combustion unit 20 vertically through combustion vortex 6 where it is combusted. As bio-fuel 2 is consumed in combustion, the thermal energy, exhaust gases and combustion by-products are carried upward in the now super heated airflow vortex 6. As the super heated air flow vortex 6 rises above burn pot 30, it expands in expansion area 53 to disperse the thermal energy across the bottom of the primary heat exchanger before being vented from combustion unit 10.

In more detail, the process begins with the pelletized or shredded bio-fuel 2 being deposited into hopper area 73. Gravity feeds bio-fuel 2 under partition wall 74, which collects in the bottom of metering area 75. Scoop arm 80 rotates to pass through the bio-fuel spilled into the bottom of metering area 75 and collect a small amount of bio-fuel in scoop bucket 82. Scoop arm 80 rotates around to deposit the collected bio-fuel into the open end of feed tube 78. The bio-fuel slides down feed tube 78 and falls vertically through central feed passage 21 of combustion unit 20. The amount of bio-fuel deposited is metered by the speed of scoop arm 80.

As shown in FIGS. 4 and 11, inlet fan 92 blows inlet air 4 through inlet duct 48 into preheat module 40. Inlet air 4 passes through preheat module 40 guided by spiral baffle 42. Inlet airflow 4 surrounds burn pot 30 and is venting into the burn pot through the plurality of holes 33. As the inlet air 4 moves through preheat module 40, the velocity of the air flow increases as the radius of spiral baffle 42 decreases toward central opening 41 and burn pot 30. As inlet air 4 travels through preheat module 40 it has been heated to more than twice the combustion temperature of the bio-fuel. The angles of holes 33 in burn pot 30 vent inlet airflow 4 into burn pot 30 at tangential angles around its circumference. The design of the spiral baffle in the preheat module and venting the inlet air flow through holes 33 has the net effect of creating a directionally forced air flow injection system for burn pot 30.

The forceable and directional venting of inlet air flow 4 into the burn pot created by the angle of holes 33 and the movement of the inlet air flow through preheat module 40 creates two distinct phenomena in the burn pot 30. First, the directional venting of the inlet air flow 4 into the burn pot 30 creates a combustion vortex 6, i.e, a cyclone of swirling inlet air and combusted gases at the center of burn pot 30 where the bio-fuel is burned. As shown, bio fuel 2 falls directly into burn pot 30 vertically through the center of combustion vortex 6 where it is ignited and consumed. Combustion vortex 6 continues to spiral upward and out of burn pot 30. The spiraling preheated inlet air inside the combustion vortex 6 surrounds and envelops the falling bio-fuel as it burns in clean and efficient combustion. Second, the directional venting of the inlet air creates an insulating swirling curtain of inlet air along the inside of wall 32. This swirling curtain of inlet air acts as a protective thermal barrier centering the combustion vortex within the burn pot and spacing the super heated combustion vortex rom sidewalls 32 of burn pot 30. Because the directional venting of the inlet air flow creates a curtain of air along inside of the burn pot, the combustion of bio-fuel is maintained in the center of the burn pot. While the temperatures at the center of the combustion vortex can reach 4000° F., the swirling curtain of inlet air maintains the temperature along the inside of wall 32 of burn pot 30 below 1500° F.

It should also be noted that a portion of inlet airflow is channeled through vent slots 45, which vent that portion of the inlet airflow into the bottom of burn pot 30. Inlet airflow 4 vented from the bottom of burn pot 30 helps suspend bio fuel 2 within burn pot 30 swirling around within combustion vortex 6. Because combustion vortex 6 prevents bio fuel 2 from settling at the bottom of burn pot 30, the bio fuel is fully consumed in the combustion process. The thermal energy released from the combustion of bio fuel 2 along with the resultant exhaust gases and by-products expands and rises within combustion vortex 6. As combustion vortex 6 rises above burn pot 30 into expansion area 53, it expand outward and across the bottom of heat exchanger 60. Heat exchanger 60 transfers the thermal energy from the superheated airflow vortex 6 into the transfer medium of the heat distribution system (not shown). Exhaust fan 94 draws exhaust air 8 from expansion area 53 through outlet duct 58 and vents it to a chimney or other exhaust duct work (not shown).

The thermal energy from the combustion of bio-fuel inside combustion chamber 31 is transferred through the walls of burn pot 30 and preheat module 40 to preheat inlet airflow 4 immediately before being vented into the burn pot. As inlet airflow 4 spirals through preheat module 40, the inlet air flow is heated to a temperature close to or above the ignition temperature of the bio-fuel. The oxygen rich, preheated inlet airflow 4 further ensures that the falling bio-fuel 2 completely combusts without settling on the bottom of burn pot 30.

Controller 90 regulates the operation of furnace 10 in relation to the heating demands of the particular application. Controller 90 regulates the revolutions of scoop arm 80 to meter the amount of bio fuel 2 deposited into combustion unit 20 and coordinates the operation of inlet fan 92 and exhaust fan 94 to regulate the air flow into and out of the combustion unit. Controller 90 monitors several variables, including combustion temperatures to precisely balance the metering of bio-fuel 2 and the regulation of the airflow through the combustion unit to maximize the transfer of thermal energy into the heat distribution system, as well as, maximizing the efficiency of the combustion.

FIGS. 15-17 show a second embodiment of a combustion unit, which is designated generally as reference numeral 100. Combustion unit 100 is substantially identical to that of previous embodiment 20, except that the burn pot 30′ has an open bottom and a fuel by-product handling assembly 110 mounted under the burn pot. A support bracket 112 is suspended under platform 26′. Two stainless steel rollers 114 are mounted axially side-by-side within bracket 112. Rollers 114 are operably connected by meshing gears 116 so that the rollers rotate in opposite directions. A drive shaft 118 extending from one of rollers 114 is connected to an electric drive motor 120 mounted to the rear of outer casing 22′. Drive motor 120 is wired to controller 90, which also controls the rotation of rollers 114. As shown, rollers 114 are positioned directly under burn pot 30′, so that any non-combustible debris and un-combusted bio-fuel 2′ falls onto the rollers. While any remaining bio-fuel 2′ will burn atop rollers 114, controller 90 intermittently rotates the rollers to crush any debris and remaining bio-fuel into ash and dust, which collects in the bottom of combustion unit 100. The collected ash and dust can be manually removed through access door 28′ or alternatively, a duct (not shown) can be connected to exhaust duct 58′ so that exhaust fan 96′ can evacuate crushed debris with the vented exhaust air.

One skilled in the art will note several advantages of the present invention. The bio-fuel furnace of this invention can burn any biomass, which can be granualized, shredded or pelletized. The furnaces burn so efficiently and at such high combustion temperatures that even bio-fuel with high moisture content can be burned without extinguishing the flame. Furnaces can burn bio-fuels with moisture content above twenty-five percent (25%). In addition, furnaces can burn bio-fuels with a high content of inert and non-combustible contaminants. Small amounts of contaminants, such as sand and bits of rock, metal and glass, are easily consumed in the combustion vortex and carried off with the exhaust gases. Because the bio fuel is generally suspended within the combustion airflow vortex, inert debris does not alter the combustion process, unlike other conventional bio-fuel furnaces. The by-product of the combustion process in the furnace is only small amounts of elemental ash. The vast majority of the elemental ash is vented with the exhaust air. Consequently, the furnace greatly reduces the maintenance required by conventional bio-fuel furnaces in disposing of collected ash.

The furnace can be readily adapted for use with any HVAC application. The furnace uses a modular design, which allows for the use of standard fittings, connectors, fans, motors, switches, wiring and controllers and allows for individual components and parts to be readily replaced, repaired or adapted for different applications. Nearly all of the components, modules and assemblies can be repaired or replaced by users with basic tools and without specialized knowledge or skill. The feed unit with the rotating scoop arm mechanism provides a maintenance free fuel delivery mechanism and eliminates many problems of conventional auger fed bio-fuel furnaces. The rotating scoop arm meters bio-fuel into the combustion unit at a controlled and reliable rate. Because of the two section design of the bin in the feed unit, the scoop arm passes through only a small portion of the stored bio-fuel for greater mechanical efficiency than auger fed systems. As a result, smaller more efficient drive motors can be used further reducing the weight, complexity and cost of the furnaces. The contact switches and variable speed drive motors wired to the controller regulate the rotation of the scoop arm to precisely meter the bio-fuel into the combustion unit. The modular design further allows for supplemental heat exchanges to be fitted within the combustion unit. For example, a supplemental heat exchanger for the production of domestic hot water or steam can be fitted behind the primary heat exchanger. The electrical fans, motors, and control electronics can be powered by a low voltage DC electrical source, such as batteries and solar cells, eliminating the need for an AC power line. While the furnace is very compact with the stacked combustion unit and the feed unit smaller than the average household refrigerator, the furnace can be upscaled for use in commercial or industrial HVAC applications. Consequently, the furnace is well suited for use in military and humanitarian applications in remote or isolated locations. The compact size, low weight and rugged operation means that the furnace can be air dropped into any location and setup to burn any available biomass at that location. The furnace eliminates the need for transporting fuel along with the furnace.

The furnace's ability to burn a wider range of bio-fuels, as well as, burn bio-fuels with high moisture and non-combustible content is partly the result of preheating the inlet air flow and creating an airflow vortex within which the bio-fuel is combusted. The preheat module uses a portion of the thermal energy from the combustion to preheat the inlet air flow above the ignition point of the bio-fuel. The directional venting of inlet air flow into the burn pot created by the angle of the burn pot holes and the accelerated movement of the inlet air flow through preheat module creates the combustion vortex within the burn pot, which surrounds and envelops the bio-fuel, which falls directly into the burn pot and vertically through the combustion vortex. The preheated oxygen rich inlet airflow spiraling in the combustion vortex provides an ideal environment for efficient and complete combustion. The combustion vortex is so strong that the falling bio-fuel is completely consumed before it settles at the bottom of the burn pot. Because the combustion is so efficient, only elemental ash and gases are produced. The elemental ash and gases are taken upward within the combustion vortex and vented eventually through the exhaust duct. The thermal energy released in this combustion process is so great that the bio-fuel often begins to combust once it enters the expansion area and reaches the super heated combustion vortex. The combustion is so efficient that often the bio-fuel is completely consumed before reaching the bottom of the burn pot. Any inert matter is blown into the combustion vortex and vented from the combustion unit with the exhaust air.

The construction and configuration of the combustion unit also provides several practical advantages. The forced directional injection of the inlet airflow into the burn pot, which creates the swirling curtain of inlet air along the inside of the burn pot helps thermally insulate the walls of the burn pot from the extremely high combustion temperatures within the combustion vortex inside the burn pot. Consequently, the burn pot does not need to be constructed from costly high temperature materials and can be constructed from less costly materials with much lower melt points. The composition and configuration of the expansion bowl allows a more efficient transfer of thermal energy to the primary heat exchanger. Because the expansion chamber is constructed of the thermal insulating material, ACC, no thermal energy is lost through conduction by the expansion chamber, itself. The concave configuration of the expansion chamber allows the vortex to expand across the bottom of the primary heat exchanger. Expanding the vortex transfers more thermal energy over a greater area of the heat exchanger, thereby reducing hot spots. The bottom of the primary heat exchanger covers the entire expansion area to provide more surface area through which thermal energy can be conducted. The internal baffles of the heat exchanger further provide surface area for conducting thermal energy from the combustion vortex into the transfer medium circulating through the primary heat exchanger. It should be noted that the heat exchanger is designed and intended to be with any transfer medium, whether air in a forced air heating application or water in hydronic floor heating applications. The design of the heat exchanger is also flow direction independent, so that either port can be used as an inlet and outlet. The ports also allow the heat exchanger to be connected to the heat distribution system using standard plumbing fittings. The use of aerated concrete as an insulating thermal lining in the combustion units greatly reduces the weight, as well as improving the structural integrity and thermal efficiency of the furnace. While other thermal insulating materials may be used, aerated concretes provide necessary insulating properties to accommodate high combustion temperatures generated in the combustion vortex. Aerated concretes have very high thermal insulation properties. The thermal lining ensures that the outer housing remains cool to the touch. Aerated concrete is also readily formed and worked, which makes it well suited for use as the composition material for the expansion bowl.

The embodiments of the present invention herein described and illustrated are not intended to be exhaustive or to limit the invention to the precise form disclosed. They are presented to explain the invention so that others skilled in the art might utilize its teachings. The embodiment of the present invention may be modified within the scope of the following claims. 

1. A bio-fuel burning furnace comprising: a bio-fuel combustion unit; and a bio-fuel feed unit mounted above the combustion unit, the combustion unit includes a housing defining an interior therein; a pre heat module disposed within the housing interior for preheating an inlet airflow to a temperature above the combustion temperature of the bio-fuel, the preheat module defining an enclosed interior therein and having a top opening therein, a burn pot seated within the pre-heat module top opening and having an interior with an open top within which the bio-fuel is burned, the burn pot having a plurality of holes therein for directionally venting the pre-heated inlet air into the burn pot so as to create a combustion vortex of inlet air, combusting bio-fuel, exhaust gases and thermal energy within the burn pot interior, a fan for blowing inlet air through the preheat module into the burn pot, the bio-fuel feed unit includes means for depositing the bio-fuel into the combustion unit so that the bio-fuel falls vertically through the combustion vortex inside the burn pot interior.
 2. The furnace of claim 1 wherein the plurality of holes in the burn pot are also spaced so as to create a curtain of swirling inlet air along the inside of the burn pot sidewall to acts as a thermal barrier between the burn pot sidewall and the combustion vortex.
 3. The furnace of claim 2 wherein the burn pot has a cylindrical sidewall, the plurality of holes in the burn pot extending through the burn pot sidewall at approximately a 30 degree angle with respect to the radius of the burn pot sidewall, and spaced around the circumference of the burn pot sidewall.
 4. The furnace of claim 1 wherein the preheat module includes an internal baffle spiraling around the burn pot for restrictively channeling inlet air toward the burn pot.
 5. The furnace of claim 1 wherein the preheat module is constructed of a thermal conducting material.
 6. The furnace of claim 1 wherein the combustion unit also includes a heat exchanger disposed within the housing interior above the preheat module and an expansion bowl disposed within the housing interior between the preheat module and the heat exchanger.
 7. The furnace of claim 1 wherein the expansion bowl has an opening therethrough spaced above the burn pot and a sloped annular face around the bowl opening.
 8. The furnace of claim 1 wherein the expansion bowl is constructed of a thermal insulating material.
 9. The furnace of claim 7 wherein the housing and the heat exchanger have a passage way through which bio fuel is deposited from the bio-fuel feed unit into the burn pot interior.
 10. The furnace of claim 6 wherein the combustion unit also includes a second fan for drawing exhaust gases around the heat exchanger and venting the exhaust gases from the combustion unit.
 11. A bio-fuel burning furnace comprising: a bio-fuel combustion unit; and a bio-fuel feed unit mounted above the combustion unit, the combustion unit includes a housing defining an interior therein, a pre heat module disposed within the housing interior for preheating an inlet airflow to a temperature above the combustion temperature of the bio-fuel, the preheat module defining an enclosed interior therein and having a top opening therein, the preheat module includes an internal baffle spiraling around the burn pot for restrictively channeling inlet air toward the burn pot, a burn pot seated within the pre-heat module top opening and having an interior with an open top within which the bio-fuel is burned defined by a cylindrical sidewall, the burn pot having a plurality of holes extending through the burn pot sidewall at approximately a 30 degree angle with respect to the radius of the burn pot sidewall, and spaced around the circumference of the burn pot sidewall for directionally venting the pre-heated inlet air into the burn pot so as to create a combustion vortex of inlet air, combusting bio-fuel, exhaust gases and thermal energy within the burn pot interior, the plurality of holes in the burn pot also spaced so as to create a curtain of swirling inlet air along the inside of the burn pot sidewall to act as a thermal barrier between the burn pot sidewall and the combustion vortex, a heat exchanger disposed within the housing interior above the preheat module, an expansion bowl disposed within the housing interior between the preheat module and the heat exchanger, the expansion bowl has an opening therethrough spaced above the burn pot and a sloped annular face around the bowl opening, the housing and the heat exchanger have a passage way through which bio fuel is deposited from the bio-fuel feed unit into the burn pot interior, a first fan for blowing inlet air through the preheat module into the burn pot, and a second fan for drawing exhaust gases around the heat exchanger and venting the exhaust gases from the combustion unit, the bio-fuel feed unit includes means for depositing the bio-fuel into the combustion unit so that the bio-fuel falls vertically through the combustion vortex inside the burn pot interior.
 12. A method of burning bio-fuel in a furnace, comprising the following steps: a. burning the bio fuel inside a burn pot; b. transferring thermal energy from the combustion of bio-fuel within a burn pot into a preheat module; c. circulating an inlet air flow through the preheat module to preheat an inlet air flow to the burn pot above the combustion point of the bio-fuel; d. directionally venting the preheated inlet air flow from the preheat module into the burn pot so as to create a combustion vortex of preheated inlet air, combusting bio-fuel, exhaust gases and thermal energy within the burn pot; e. depositing additional bio-fuel into the burn pot such that the bio-fuel falls vertically through the combustion vortex whereby the bio-fuel is burned within the combustion vortex.
 13. The method of claim 12 and an additional step: f. expanding the combustion vortex across the bottom of a heat exchanger to uniformly conduct thermal energy from the combustion vortex to the heat exchanger.
 14. The method of claim 12 wherein step d includes venting the preheated inlet air flow through a plurality of angled bores formed within the burn pot.
 15. The method of claim 12 wherein the transferring of thermal energy of step b is preformed by conducting and radiating thermal energy through the burn pot into the preheat module. 